6 VERTEBRATE SKELETON 



history. The other, cartilage bone, is preformed in cartilage, which 

 is later torn down, its organic substance (chondrin) being replaced 

 by ossein with a large amount of lime salts. The distinction 

 between the two kinds of bone is of great importance in determining 

 homologies (or their lack) in bones occupying the same relative posi- 

 tions in different Vertebrates. 



Cartilage begins by an accumulation of mesenchyme cells, its 

 early stage (procartilage) being readily recognized in the stained 

 section by the closely packed nuclei with a small amount of proto- 

 plasm around them. As development proceeds each cartilage cell 

 secretes chondrin around itself, the amount of this increasing with 



time, so that eventually the cells are sep- 

 arated by a considerable amount of chondrin, 

 the so-called matrix. This process increases 

 the size of the cartilage mass, the growth of 

 which is also effected by the division of the 

 cartilage cells as well as by the addition of 

 cells from the surrounding mesenchyme. 

 When a cartilage element has attained its 

 Fig. 5— a bit of calcified definitive form, the mesenchyme touching it 



cartilage, with'radiatingspicu- , j~, j_- j_- t • 



les of calcic carbonate. becomcs a fibrous conncctivc tissuc, formmg 



an envelope (perichondrium), which con- 

 nects adjacent cartilages, affords attachment for muscles and 

 ligaments and carries blood vessels to nourish the cartilage, although 

 no vessels enter the matrix. 



In many cases the cartilage thus formed remains unmodified and 

 constitutes the hyaline cartilage of histology. Or it may be 

 strengthened by fibres (elastic or non-elastic) in the matrix. In 

 adult Elasmobranchs, and especially the larger species, a deposition 

 of lime salts occurs in the matrix, this being most marked in the later 

 and outer layers, the result being calcified cartilage, most abundant 

 on the outer surface of the skull, vertebrae, etc. (fig. 5). Calcified 

 cartilage differs from bone in the absence of canals and canaliculi, 

 in having no blood vessels in the matrix and in not having the cells 

 in layers (lamella?), while the calcareous salts form irregular patches 

 of radiating spicules (fig. 5). In Myxinoids different kinds of carti- 

 lage — true and pseudo — each with hard and soft varieties — are recog- 

 nized, for details of which reference should be made to special 

 papers. 



